add SPI slave support

Set up SPI slave transactions for entries in the SPI send queue.
Add a header to each SPI datagram that includes a CRC16,
the type and the size of the message that follows.

Does not act on received bytes (yet).

Signed-off-by: Christian Ambach <christian.ambach@deutschebahn.com>

src/hal/generic.h

@@ -7,6 +7,11 @@
 
 #define HAS_LORA 1 // comment out if device shall not send data via LoRa or has no LoRa
 #define HAS_SPI 1  // comment out if device shall not send data via SPI
+// pin definitions for SPI slave interface
+#define SPI_MOSI GPIO_NUM_23
+#define SPI_MISO GPIO_NUM_19
+#define SPI_SCLK GPIO_NUM_18
+#define SPI_CS GPIO_NUM_5
 
 #define CFG_sx1276_radio 1 // select LoRa chip
 //#define CFG_sx1272_radio 1 // select LoRa chip

src/hal/lopy4.h

@@ -7,6 +7,12 @@
 
 #define HAS_LORA 1       // comment out if device shall not send data via LoRa
 #define HAS_SPI 1        // comment out if device shall not send data via SPI
+// pin definitions for SPI slave interface
+#define SPI_MOSI GPIO_NUM_22
+#define SPI_MISO GPIO_NUM_33
+#define SPI_SCLK GPIO_NUM_26
+#define SPI_CS GPIO_NUM_36
+
 #define CFG_sx1276_radio 1
 //#define HAS_LED NOT_A_PIN // LoPy4 has no on board mono LED, we use on board RGB LED
 #define HAS_RGB_LED (0)  // WS2812B RGB LED on GPIO0 (P2)